This invention relates generally to a capacitive, operator-sensing circuit for disabling a component, such as an internal combustion engine, of mobile power equipment and, more particularly, a capacitive, operator-sensing circuit including a charge-transfer sensor coupled to a capacitive sensing electrode affixed to a gripping surface of the mobile power equipment, the circuit functioning to disable the component upon sensing a change in the charge of the sensing electrode as a result of an operator removing his or her hand from the gripping surface.
Various types of mobile power equipment driven by internal combustion engines are in widespread use today such as lawn and garden maintenance equipment, construction equipment, and agricultural implements. Examples of such internal combustion mobile power equipment include power lawnmowers (push and self propelled), rototillers, cultivators, snowblowers, power lawn edgers, riding lawnmowers, garden tractors, etc. Such equipment includes a handle or gripping surfaces which is/are used by the operator to push the equipment (if not self-propelled) and/or direct the equipment movement along a desired path of travel. Gripping surface might include a steering wheel and steering wheel spokes of a riding lawnmower or the steering levers of a skid steer vehicle. A constant concern with such mobile power equipment is insuring that the engine is disabled (turned off) in the event that the operator losses control of the equipment, i.e., the operator losses his or her grip on the handle or gripping surfaces. In some types of mobile power equipment a working implement, such as a cutting blade or an auger, is driven by the motor and there is an electrical or mechanical clutch disposed between the engine, a transmission and the working implement. In such equipment instead of turning off the engine, it may be sufficient to disengage the clutch when the operator losses his or her grip on the handle. Disengaging the clutch insures the working implement is no longer driven by the engine even though the engine continues to run. Such a clutch arrangement is used in equipment having a working implement driven by a power take-off drive system. A power take-off drive system typically includes a drive shaft to which the working implement (cutting blade, auger, etc.) is attached. When the power take-off drive is engaged, the engine rotates the power take; off drive shaft.
Various types of mechanical assemblies have been used on such mobile power equipment to shut off the engine and/or disengage the power take-off drive system in the event that the operator losses his or her grip on the handle or gripping surfaces. Generally such mechanical assemblies included a lever or rod that is required to be pivoted to a position adjacent, for example, the handle for the engine to operate. The operator thus grips both the lever and the handle simultaneously to maintain the lever adjacent the handle. The lever is typically biased by a spring to swing away from the handle if the operator releases his or her grip on the handle thereby shutting off the engine and/or drive wheels on self-propelled mobile power equipment and/or disengage a power take-off drive shaft.
Such mechanical assemblies have several disadvantages including being costly to install and maintain, adding additional weight to the equipment and continuously exerting pressure against the operator""s hand causing operator fatigue. Additionally, such mechanical assemblies are capable of being defeating by simply tying the lever to the handle with a piece of rope or twine.
An electrical operator-sensing circuit would overcome many of the disadvantages of a mechanical assembly including fatigue and defeatability. One such electrical circuit proposal is disclosed in U.S. Pat. No. 4,145,864 to Brewster, Jr. The ""864 patent discloses an operator sensing circuit that requires an operator to electrically bridge first and second spaced apart operator contacts mounted on the lawnmower handle of an electric lawnmower for actuation of the electric motor that drives the cutting blade. However, the ""864 patent requires that both hands of the operator be on the handle. It is often desirable and necessary to permit continued operation of the equipment if the operator maintains one hand on the handle, e.g., the operator removes one hand from the handle to wipe his or her brow or push low hanging branches out of the way when mowing under a tree. Additionally, the ""864 patent teaches the use of a phase lock loop circuit requiring a signal generator and signal receiver which is relatively complex and expensive.
What is needed is a simple-to-fabricate and install, low-cost, rugged, durable and difficult to defeat operator-sensing circuit which permits continued operation of the power equipment when at least one hand of the operator is maintained on the equipment handle.
A capacitive, operator-sensing circuit for mobile power equipment includes a charge-transfer touch sensor that is electrically coupled to at least one capacitive sensing electrode affixed to a gripping surface of the mobile power equipment. The charge-transfer sensor periodically generates charge bursts that are coupled to the sensing electrode and a floating transfer capacitor. The charge-transfer sensor senses the charge buildup on the sensing electrode resulting from the charge bursts. The sensor changes an output signal between a first and a second states if the sensed charge changes by a predetermined threshold magnitude.
The charge-transfer sensor is preferably a digital integrated circuit that operates to change an output voltage signal between two levels or states depending upon sensed charge of the sensing electrode. The gain and sensitivity of the sensor are configured such that the sensed charge of the sensing electrode changes by the predetermined threshold magnitude (or more) when an operator""s hand is moved from a position of gripping or touching the gripping surface to a position of not gripping or touching the surface. The charge-transfer sensor outputs a DC voltage signal, maintaining a given output voltage level until the sensor senses a change in the sensed charged of the predetermined threshold value.
The operator-sensing circuit additionally includes coupling circuitry coupled between the charge transfer touch sensor and a component or system of the mobile power equipment. For example, the component may be an internal combustion engine of the equipment and the coupling circuitry may be coupled to a magneto of the internal combustion engine. A change in sensor output voltage signal from a first level to a second level causes the engine magneto to be electrically coupled to ground thereby shutting off the internal combustion engine.
The coupling circuit includes a first and second field effect transistors (FETs) coupled to a gate of a triac bilateral switch. The triac is coupled between the magneto and ground and, when the break over threshold voltage of the triac is exceed by application of a sufficiently high positive or negative voltage to the triac gate terminal, the triac conducts in both forward and reverse direction as necessary to ground the magneto and shut off the engine.
In one preferred embodiment of the operator-sensing circuit of the present invention, the charge-transfer sensor is coupled to two sensing electrodes which are disposed at spaced apart positions on the equipment handle or gripping surfaces. While either or both of the sensing electrodes is contacted by an operator""s hand, the sensor output voltage signal remains at the first level and the engine magneto is not grounded out.
In another preferred embodiment of the operator-sensing circuit of the present invention, the coupling circuit includes a relay which switches between two open and closed conditions as the sensor output voltage changes levels (logic high and logic low). The relay may advantageously be electrically coupled to a controller which disengages a component of the mobile power equipment such as a power take off drive when the operator""s hands are removed from the sensing electrodes. Alternately, in mobile power equipment driven by an electric motor, such a coupling circuit utilizing a relay and controller may be advantageously used disable or turn off power to the electric motor, when the operator removes his or her hands from the sensing electrodes.
These and other objects, advantages, and features of an exemplary embodiment of the present invention are described in detail in conjunction with the accompanying drawings.